This invention relates to a molded part excellent in mechanical and electrical characteristics which is particularly suitable as an endless belt.
Endless belts are widely used in image forming apparatus (e.g., OA equipment) as a photoreceptor belt, an intermediate transfer belt, a carrier transfer belt, a transfer separation belt, a charged tube, a developing sleeve, a fixing belt, a toner transfer belt, and the like.
For example, an intermediate transfer system comprises an intermediate transfer member, on which a toner image is formed and transferred onto a recording medium, such as paper. An endless belt is used as the intermediate transfer member for electrifying and destaticizing a toner on its surface. The electrical resistance of the endless belt to be used is set according to the model of the system. A carrier transfer system comprises a carrier transfer member, on which paper is held. After a toner is transferred from a photoreceptor onto the paper held on the carrier transfer member, the paper is separated from the carrier transfer member by destaticization. An endless belt is used therein as a surface of the carrier transfer member for electrifying and destaticizing paper. The electrical resistance of the endless belt for the carrier transfer system is also set for individual models.
An electrically conductive endless belt used in electrophotographic copying machines, etc. is driven for a long period of time under high tension by two or more rolls and is therefore required to have sufficient durability. Where used in the intermediate transfer system, etc., since a toner image is once formed on the belt and then transferred to paper, a sag or stretch of the belt in operation results in image distortion. Further, the belt used in the intermediate transfer system is required to have electrical conductivity to some extent to carry out electrostatic toner transfer.
With the increasing printing speed of latest printing machines, the belt driving speed has been increasing, which demands further improvement in durability of the belt. The belt durability and prevention of image distortion are particularly demanded for the belts to be used in tandem type carrier transfer or intermediate transfer systems having four photoreceptors and toner jetting systems which are attracting attention for their high-speed printing ability.
Various molding materials which have been employed to produce the above-described endless belts have their several disadvantages as follows.
Endless belts made of rubber are too stretchy due to their low elastic modulus and have poor toner releasability. Laminating rubber with other materials has been attempted, which makes the production process complicated and increases the cost.
Endless belts made of polycarbonate (hereinafter abbreviated as PC) have poor flex resistance and are liable to develop cracks while driven on rollers.
Endless belts made of polyimide are, while excellent in flex resistance, not only difficult to produce by continuous molding because polyimide is a thermosetting resin but expensive. Further, their elastic modulus are as high as about 6000 MPa, which imposes a load to the belt driving motor. The load cannot be reduced but by reducing the belt thickness, but such will lead to poor reliability because the belt with a reduced thickness would easily develop cracks if dust enters between a roller and the belt, or if the belt is scratched by friction with the photoreceptor.
Endless belts made of a polyalkylene terephthalate (hereinafter abbreviated as PAT) are better than PC belts in flex resistance but leave much room for improvement.
Endless belts made of a fluororesin is, while excellent in flex resistance, stretchy under tension due to their low Young""s modulus of about 1000 to 1400 MPa. As a result, color print may suffer from a shear, or the toner may be transferred as distorted to paper.
Endless belts made of a polyblend of PC and a PAT, such as polybutylene terephthalate, are also proposed (see JP-A-4-313757 and JP-A-6-149083). They have improved flex resistance over PC belts but are still insufficient. Besides, a PAT resin, being highly crystalline, adversely affects the dimensional precision of the endless belt if used in an increased proportion.
An electrically conductive filler, such as carbon black, is often incorporated into a molding resin to produce an conductive endless belt. However, the incorporated filler tends to reduce the mechanical properties.
Although various endless belts have hitherto been proposed and put to practical use as stated above, an endless belt satisfactory in mechanical properties such as flex resistance and Young""s modulus, electrical properties such as resistance, as well as economy is unknown.
Various thermoplastic resins have been used in the structural or functional parts of OA equipment, which are exemplified by the aforementioned endless belt, the exterior and interior parts of automobiles, the structural parts of appliances, and the like. The thermoplastic resins for these applications are required to have a high elastic modulus, excellent flex resistance, excellent chemical resistance, dimensional precision, and, for some uses, transparency. Thermoplastic resins are roughly divided into crystalline ones and amorphous ones. In general, crystalline thermoplastic resins are excellent in flex resistance and chemical resistance but poor in dimensional stability because of their high mold shrinkage coefficient and have no transparency. On the other hand, amorphous thermoplastic resins are excellent in dimensional stability on molding and transparency but poor in chemical resistance.
To meet all the requirements for flex resistance, chemical resistance and dimensional stability on molding, polyblends of crystalline thermoplastic resins and amorphous thermoplastic resins have been studied for improvement on these properties. In the field of thermoplastic ester resins, for example, it has been reported that a crystalline ester resin and an amorphous ester resin can be finely dispersed mutually by accelerating interesterification (copolymerization). However, this technique has not yet been put to practical use for the following reasons. Firstly, accelerated interesterification (copolymerization) is accompanied by depolymerization to produce low-molecular weight components, which cause foaming on molding. Secondly, molecular chain cutting will proceed to reduce the molecular weight, which leads to reduction of mechanical characteristics (e.g., reduction of breaking extension) of the molded part. Suppressing the interesterification to prevent reduction in physical properties has been studied but not succeeded in providing a molded part excellent in both elastic characteristics (high elastic modulus) and flex resistance.
An object of the present invention is to provide a molded part, especially an endless belt, which has high elasticity and excellence in flex resistance, chemical resistance and dimensional stability on molding and is free from physical properties deterioration due to reaction during melt mixing.
Polyblends of thermoplastic resins are roughly divided into compatible ones and incompatible ones. Incompatible polyblends assume a sea-island structure because two thermoplastic resins are not completely mixed by melt blending. Where the two thermoplastic resins have largely different volume fractions, the one having a larger volume fraction is apt to form a continuous phase (sea) while the other is apt to form a discontinuous phase (islands). Where the difference of volume fraction is small, the difference in melt viscosity is influential on the disperse structure, and the one having a smaller melt viscosity tends to form a continuous phase while the other tends to form a discontinuous phase. Having of necessity interfaces between the continuous and the discontinuous phases, the incompatible polyblend is most liable to break at the interfaces, which seems to reduce flex resistance. Hence the present inventors have thought that flex resistance of a molded part would be improved by strengthening the interfaces by graft polymerization, block polymerization and the like.
On the other hand, the compatible polyblend usually has no interfaces between different resins, i.e., a non-sea-island structure. It is said that the physical properties, such as mechanical properties typified by flex resistance, of the compatible polyblend agree with the blending ratio of the two resins. Therefore, polyblending is not expected to bring about improvement on flex resistance. The present inventors considered that mechanical properties such as flex resistance would be improved greatly if the two materials are compatible with each other and also copolymerized. In other words, it is expected that a copolymerized state in a thermoplastic polyblend, whether compatible or incompatible, would improve the mechanical properties such as elastic modulus and flex resistance of the polyblend.
The inventors have paid their attention to polyester resins, which are inexpensive thermoplastic resins. It is known that polyesters, when blended, generally undergo mutual molecular cutting and exchanging called interesterification. The interesterification is said to generally involve molecular chain cutting to cause foaming or brittleness, resulting in reduction of mechanical characteristics. However, the inventors have thought that the interesterification reaction must be concurrently accompanied with copolymerization-like reaction and assumed that specific conditions would lead the interesterification reaction to copolymerization reaction while suppressing reduction of mechanical characteristics due to molecular chain cutting. Based on this assumption, the inventors have continued their study on the kinds and compositions of materials to be blended, the molding conditions and the like.
As a result, the following facts have been revealed. Firstly, a polyester resin mixture can produce (1) a polyblend having a sea-island structure, (2) a polyblend having a sea-island structure with the interfaces copolymerized, and (3) a polyblend having non-sea-island structure. The mechanical properties typified by flex resistance increase in the order of (1), (2) and (3). Addition of a third component such as a conductive filler to (2) (sea-island structure with copolymerized interfaces) or (3) (non-sea-island structure) induces little reduction in mechanical strength.
Secondly, where a conductive filler is added to control electrical resistance for specific uses, (2) (sea-island structure with copolymerized interfaces) or (3) (non-sea-island structure) has the filler fixed to the copolymerized resinous components and therefore exhibits electrical stability and hardly undergoes change in resistance with time.
Thirdly, (1) (sea-island structure) and (2) (sea-island structure with copolymerized interfaces) hardly stretch in a broad range of temperature and maintain a high elastic modulus and creep resistance even in high temperature or with passage of time for several days to as long as several years.
Based on these findings, the inventors have attested that a molded part obtained from a molding material prepared by mixing a crystalline resin, an amorphous resin, and a polymerization catalyst while heating is superior to one obtained by any of conventional techniques comprising control on interesterification reaction, etc. in terms of physical properties such as flex resistance. In addition, they have found that the molded part can possess transparency under some conditions.
The present invention provides a molded part obtained by mixing a crystalline resin having at least one of a hydroxyl group, a carboxyl group and an ester linkage, an amorphous resin having at least one of a hydroxyl group, a carboxyl group and an ester linkage, and a polymerization catalyst while heating to prepare a resin composition and molding the resin composition.